Neovascular formation in adults has been thought to result exclusively from proliferation, migration, and remodeling of preexisting endothelial cells (ECs), a process referred to as angiogenesis. (Folkman J. Nat Med 1995; 1:27-30; Schaper W et al. Circ Res. 1971; 28:671-679; Risau W. Nature, 1997, 386:671-674) In contrast, vasculogenesis, a process defined as the formation of new blood vessels from endothelial progenitor cells (EPCs) during embryogenesis (Risau W. Nature 1997, 386:671-674: Risau W. FASEB J. 1995; 9:926-933; Risau W et al. et al. Development, 1988; 102:471-478), begins by the formation of blood islands that comprise EPCs and hematopoietic stem cells (HSCs). (Flamme I et al. Development 1992; 116:435-439 Hatzopoulos et al. Development. 1998; 125:1457-1468) Blood islands fuse with each other to create primordial vascular networks in the embryo. EPCs and HSCs are believed to originate from common mesodermal ancestral cells (i.e., hemangioblasts) because of the presence of common cell surface antigens, such as Flk-1/KDR, Tie-2, and CD34. (Millauer B et al. Cell. 1993; 72:835-846; Sato TN et al. Nature. 1995; 376:70-74; Yano M et al. Blood. 1997; 89:4317-4326; Krause DS et al. Blood. 1996; 87:1-13).
Recently, circulating EPCs have been discovered in adult peripheral blood and human umbilical cord blood. (Asahara T et al. Science 1997; 275:964-967; Murohara T et al. J. Clin Invest, 2000; 105:1527-1536.) Circulating EPCs have been shown to participate in postnatal neovascularization after mobilization from bone marrow (BM). (Takahashi et al. Nat. Med. 1999; 5:434-438) Moreover, in an earlier study, Noishiki et al. raised the possibility of facilitating luminal endothelialization and mural angiogenesis in an artificial vascular prosthesis by BM transplantation. (Noishiki Y et al. Nat Med. 1996; 2:90-93) Shi et al. recently showed that BM cells mobilized and participated in endothelialization of implanted artificial vascular grafts. (Shi Q et al. Blood. 1998; 92:362-367) Although these studies suggest that EPCs originate from BM in adults, little is known as to whether functional EPCs can develop from adult BM cells and whether transplantation of autologous BM can quantitatively and functionally augment neovascular formation in ischemic tissues in adult species. These issues are relevant, because therapeutic angiogenesis is an emerging strategy to salvage tissues from critical ischemia. (Baumgartner I et al. Circulation. 1998; 97: 1114-1123; Losordo D W et al. Circulation. 1998; 98:2800-2804; Isner J M and Asahara T J Clin Invest. 1999; 103:1231-1236).
Accordingly, the studies of the present invention tested the hypotheses that (1) functional EPCs may develop from BM mononuclear cells (BM-MNCs) in adult animals and (2) transplantation of autologous BM-MNCs may augment neovascularization in response to tissue ischemia in a pig model of chronic myocardial ischemia and in a rabbit model of unilateral hindlimb ischemia.
Direct local transplantation of autologous BM-MNCs is a useful strategy for therapeutic neovascularization in ischemic tissues in adult mammals consistent with “therapeutic vasculogenesis.”